The present invention relates to a method and apparatus for improving the representation of greys in a digitised image.
It is known that any image can be represented and described as a matrix of elementary areas or pixels, to each of which is assigned one of two tones, black and white respectively, to which is associated one of two logic or electrical levels.
In this case each pixel is described and can be represented by a single bit of information.
The pixel can easily be displayed with high contrast display devices and processes (on screen or by printing).
This method of description and generation of images also lends itself, as is known, to describing and displaying images having several tone levels or greys, obtained not by modulation of the optical density of the image dots, but by local variation of the ratio between light and dark areas.
Below a threshold size the human eye integrates the perception of separate areas in view into a single area of uniform intermediate tone.
Although it is today possible, with high contrast display devices and processes to vary within the ambit of one image pixel the width of the dot represented, either black on white or white on black, and therefore modulate, pixel by pixel, the ratio between the black and white areas and thus the apparent grey level, this involves the necessity of using more information bits at the digital image description level to describe the state of each individual pixel.
For example, with two bits a pixel can be described as white, black or characterised by two different dimensions and therefore two apparent separate grey levels.
With 3 bits the possible number of tones rises to 8 (black and white included), with 4 bits to 16 and with 5 bits to 32.
Since digital display devices presuppose a digital description of the image stored on a physical support of optical, magnetic or electronic type, that is to say in a "memory", it is evident that a considerable increase in the memory capacity is required to describe a digitised image with a predetermined resolution, for example of 300.times.300 dpi (dots per inch) if each dot has to have information on the grey level associated with it.